ABSTRACT We have an ability to discriminate between millions of odors. Each odorant binds to subsets of olfactory sensory neurons, which subsequently activate specific glomeruli in the olfactory bulb (OB). In the glomeruli, mitral and tufted cells, OB projection neurons, receive synaptic inputs from the olfactory sensory neurons. Mitral/tufted cells are the only neurons that transmit olfactory information to the olfactory cortex. To process the vast amount of information from the olfactory sensory neurons, OB projection neurons consist of functionally different subpopulations. Over the past several decades, differences between mitral and tufted cells have been intensively studied. Moreover, several lines of recent evidence show that we cannot consider mitral cells as a homogeneous population anymore. However, the most fundamental aspects of mitral cell heterogeneity are still unknown: How many mitral cell subpopulations? What factor(s) determines the mitral cell properties? Are there correlations among different mitral cell properties? What are the roles of each mitral cell subpopulation in olfactory information processing? As a starting point to address these significant gaps in our knowledge, we previously demonstrated that mitral cells generated at different timing during development were incorporated into functionally distinct neuronal circuits in the OB, suggesting that we can sort the mitral cells into functionally different subgroups based on their timing of neurogenesis (?neuronal birthdate?). In this project, we will separately label the early- and late-generated mitral cells with fluorescent proteins using an innovative method developed by the PI; in utero electroporation of plasmid vectors. Using this technique, we will determine whether mitral cells with different neuronal birthdates play different roles in olfactory information processing by pursuing following three specific aims: Specific Aim 1) Determine whether the molecular expression profile of mitral cells varies based on their timing of neurogenesis; Specific Aim 2) Characterize the functional properties of early- and late-generated mitral cells; and Specific Aim 3) Examine whether early- and late-generated mitral cells differentially project axons to the olfactory cortex. The proposed project will advance our knowledge about rule(s) used for olfactory information processing in the OB and the olfactory cortex.